1. Field of the Invention
The present invention relates to a linear motor and a camera module having the linear motor, and more particularly, to a linear motor that can remove a dead zone with a simple structure, can prevent an unnecessary increase in thickness, does not cause a stoppage at the time of moving a camera lens, has a small amount of tilt to enable a linear control, and can contribute to a decrease in thickness and a compactness of a mobile terminal and a camera module having the linear motor.
2. Description of the Related Art
Mobile terminals such as mobile phones and PCS have required various functions and a decrease in size and thickness.
Particularly, with the generalized use of mobile terminals having a camera attached thereto, techniques associated with an optical lens and an image sensor for high image quality and techniques associated with a zoom function of a camera are considered as being more important.
Mobile terminals equipped with a camera having three million or more pixels, which were embodied in past digital cameras, come to the market. An auto focusing function is required for the cameras of the mobile terminals so as to satisfactorily exhibit their functions.
To additionally provide the auto focusing function to a camera of a mobile terminal, an actuator (actuation unit) moving a camera lens of the mobile terminal should be provided. The actuator needs the have small size and thickness for a decrease in size of the mobile terminal, that is, for a decrease in size and thickness of the mobile terminal. The linear movement, the power consumption, and the driving force of the camera lens should be also considered in addition to the size of the actuator.
A stepping motor or a voice coil motor enabling a linear control could be considered as the actuator for embodying the auto focusing function of the camera lens.
The stepping motor has an advantage of a precise control and a great force, but has a disadvantage of a complex structure and a great volume. The voice coil motor has an advantage of a precise control and a small volume due to the simple structure thereof, but has a disadvantage of great power consumption and weak driving force.
Accordingly, there is a need for a linear motor that can reduce the size, the noise, and the power consumption, does not cause a stoppage at the time of moving a camera lens, and has a small amount of tilt to enable a linear control. For this purpose, studies have been actively done.
FIG. 1 is a graph illustrating a displacement in a longitudinal direction of a shaft under a free-free condition where both ends are not fixed. FIG. 2 is a graph illustrating a displacement in the longitudinal direction of the shaft under a fix-free condition where only one end is fixed. FIG. 3 is a graph illustrating an elastic displacement when the shaft is not fixed. FIG. 4 is a diagram schematically illustrating a structure of a linear motor applied to a past mobile terminal.
A structure of a linear motor 105 applied to a camera module 101 of a past mobile terminal will be first described with reference to FIG. 4. The linear motor 105 has a structure in which a shaft 131 is connected to one side of a piezoelectric substrate 120 generating a vibration mode in the longitudinal direction and a moving member 140 connected to a camera lens 150 is coupled to the shaft 131. A node fixing portion 160 is further disposed in an area adjacent to the piezoelectric substrate 120.
With this structure, when a voltage is applied to the piezoelectric substrate 120 to generate a vibration mode in the longitudinal direction, the moving member 140 coupled to the shaft 131 moves in the longitudinal direction of the shaft 131 as indicated by the solid line or the dotted line in FIG. 4 to move the camera lens 150, thereby embodying the auto focusing function of the camera lens 150.
On the other hand, it has been reported with this structure that the vibration mode of the shaft 131 due to the ultrasonic vibration of the piezoelectric substrate 120 varies depending on a supporting condition of the shaft 131.
For example, under a free-free condition of FIG. 1 where both ends of the shaft are free, a node having an elastic displacement of 0 exists among many points in the shaft 131. This causes a dead zone (a section not moving) of an impact-type actuator depending on restriction conditions (indicated by an arrow in FIG. 1). Under a fix-free condition of FIG. 2 where only one end of the shaft 131 is fixed, the displacement from the fixed point increases from 0 to the other end.
On the other hand, when the shaft 131 of the impact-type actuator is not fixed, a node having an elastic displacement of 0 exists, which is located in the vicinity of the piezoelectric substrate 120 as shown in the graph of FIG. 3.
In order to remove such a node, that is, the dead zone, not to influence the movement, the node fixing portion 160 may be disposed in the vicinity of the node as shown in FIG. 1. However, since the structure shown in FIG. 1 requires an unnecessary increase in thickness (indicated by t in FIG. 1), there is caused a problem that the volume and the thickness of the mobile terminal increases.